[Coral-List] Excess algal symbionts increase coral susceptibility to bleaching
joerg.wiedenmann at noc.soton.ac.uk
Thu Nov 22 19:14:32 EST 2012
Please allow me to add some thoughts to the very interesting discussion about aeolian dust, nutrients and bleaching susceptibility.
While iron was certainly imported in the Caribbean by aeolian dust for a long time, anthropogenic activities increased the influx of nutrients rather recently. Caribbean corals were shown by Rob van Woesik and co-workers to have a higher bleaching prevalence in high productivity regions with elevated chlorophyll a and dissolved inorganic nitrogen concentrations (Wagner et al., 2010). The addition of an (exceptionally) higher iron load can be easily imagined to result in a further increase of algal productivity (Roff & Mumby, 2012; Ferrier-Pages et al. 2001). As mentioned by Andrew, the increased zooxanthellae densities might result in an increased ROS load under temperature stress (Cunning & Baker 2012). At the same time, the proliferating zooxanthellae populations have an increased demand in all essential plant nutrients. If this demand is not fullfilled, e.g. due to imbalanced concentrations in the water column, the algae can run into nutrient starvation. We have shown that phosphate starvation of zooxanthellae results in a rather dramatic increase to temperature and light induced bleaching (Wiedenmann et al 2012).
Undoubtedly, temperature and light stress are involved in many/most bleaching episodes. However, it becomes increasingly important in my opinion to consider a range of physiological conditions that render corals more susceptible to these forms of environmental stress. It might be that in one year / region corals harbouring nutrient starved zooxanthellae fall victim to bleaching at lower levels of heat / light stress then elsewhere. Under different circumstances, they might experience an increased susceptibility under temperature stress due the ROS production of an increased load of “fertilised” zooxanthellae. On other occassions, temperature and light stress alone might inflict sufficient to damage to the photosynthetic apparatus of the algae to cause bleaching. Together with other reasons for bleaching that were not discussed here, such scenarios offer explanations for the patchiness of bleaching.
I have just started a five year project as part of the ERC Starting Grant Scheme (http://erc.europa.eu/erc-funded-projects). Aside from exploring the detailed mechanisms and implications of nutrient starvation of zooxanthellae, the INCORALS project aims to identify protein and lipid biomarkers that will enable to discriminate among different physiological conditions underlying different forms of increased susceptibility to thermal stress. Together with results of the project introduced by Andrew and colleagues, the outcomes of INCORALS should hopefully facilitate the classification of bleaching events in the future.
Cunning R. and Baker A.C. (2012) Excess algal symbionts increase the susceptibility of reef corals to bleaching. Nature Climate Change advance online publication.
Ferrier-Pages C., Schoelzke V., Jaubert J., Muscatine L. and Hoegh-Guldberg O. (2001) Response of a scleractinian coral, Stylophora pistillata, to iron and nitrate enrichment. J Exp Mar Bio Ecol 259, 249-261.
Roff G. and Mumby P.J. (2012) Global disparity in the resilience of coral reefs. Trends Ecol Evol 27, 404-413.
Wagner D.E., Kramer P. and van Woesik R. (2010) Species composition, habitat, and water quality influence coral bleaching in southern Florida. MEPS 408, 65-78.
Wiedenmann J., D'Angelo C., Smith E.G., Hunt A.N., Legiret F.-E., Postle A.D. and Achterberg E.P. (2012) Nutrient enrichment can increase the susceptibility of reef corals to bleaching. Nature Climate Change doi:10.1038/nclimate1661.
PD Dr. Joerg Wiedenmann
National Oceanography Centre
University of Southampton, Waterfront Campus
Southampton, SO14 3ZH
email: joerg.wiedenmann at noc.soton.ac.uk
Mobile: +44 (0)7912564356
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